This invention relates to electric generating power plants including Compressed Air Energy Storage (CAES) systems. More particularly, this invention relates to a CAES System which stores excess power from an electric power generating plant during off-peak hours and returns stored energy as peak power when required. More particularly, this invention relates to a CAES System in which the steam and compressed air energy storage cycles are directly coupled to form a Direct Coupled Steam/Compressed Air Energy Storage Cycle (DCS/CAES).
The electric utility industry has as its main objective the supply of power at the lowest possible cost. This purpose has lead to the development of large sophisticated nuclear and fossil-fuel-fired steam generating plants. For both technical and economical reasons, these plants should be operated at a steady load. However, to meet daily and seasonal fluctuations in power demands, the industry uses so called peaking units. The most common form of these units is gas turbine systems that use premium fuels such as natural gas and oil.
Because of the limited supply of oil and natural gas in this country and because of current problems in the supply of petroleum fuel from foreign sources, premium fuel has become very expensive and the long-term supply is uncertain; therefore, electric utilities have been exploring better ways of utilizing, or even eliminating, the use of premium fuels for peaking units and the possibility of operating their large power plants at steady or constant load. These considerations have lead to the investigation of energy storage systems.
Studies conducted by electric utilities indicate that CAES power plants are attractive for consideration as an energy storage system. A CAES plant may comprise a steam system including a boiler, steam turbine, and generator, and an air system including a motor driven compressor and an air turbine connected to a generator. Prior art air compressors are typically motor driven, their energy source being the steam driven generator. Hence, reduced efficiencies are incurred in this arrangement. Further, many systems require additional motors for booster air compressors. When compressed air is cooled and stored in an underground air storage reservoir, it is available to supply energy under peak load conditions. When released, the stored air is typically heated and expanded through one or more air turbines. Some systems have included a means for storing the heat of the air as it is compressed for storage and applying the stored heat to the stored air as it is expanded. In prior art arrangements, the air turbine is not shafted to the air compressor, in contrast to conventional peaking units.
The purpose of this arrangement is two-fold. Firstly, the compressor system is driven with cheap off-peak power from the main steam power generating plant and the energy is stored as compressed gas in a compressed gas reservoir. Secondly, during peak hours, the compressed air stored in the reservoir is utilized to run the turbines at their power. This is in contrast to a conventional gas turbine peaking unit wherein the total power output is reduced by 1/3 to 2/3. The reason for this reduction in power is because that portion of the system's power is used internally to operate compressors which are needed to supply compressed air to the turbine. By utilizing a CAES system, fuel savings on the order of 50% to 60% are possible. However, the CAES systems still have the problem of incurring energy conversion losses during off-peak hours when the steam energy is converted to electrical energy, which in turn is converted to compressed air energy.
Another problem encountered in the electric utility industry, in addition to peak loads, and peak load plants, are intermediate load power plants, those plants which operate for approximately eight hours a day. Currently, new intermediate load power plants are cycling coal-fired plants, cycling oil-fired plants, or combined cycle power plants. The cycling oil-fired plants and the combined cycle plants burn petroleum or natural gas which is undesirable, and they produce very expensive power. The cycling coal plant also produces expensive power. The DCS/CAES cycle has the potential to substantially reduce the cost of intermediate load electric power.
It is, therefore, an object of the present invention to provide an improved CAES power plant which substantially eliminates the inefficiencies of energy conversion. It is another object of the present invention to provide a continuously operated intermediate load power plant, the efficiency of each component thereof being increased by directly coupling the steam and compressed air energy storage cycles.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.